Chapter Description

From the Book

Introduction to Optical Design

A network planner needs to optimize the various electrical and optical
parameters to ensure smooth operations of a wavelength division multiplexing
(WDM) network. Whether the network topology is that of a point-to-point link, a
ring, or a mesh, system design inherently can be considered to be of two
separate parts: optical system design and electrical or higher-layer system
design. To the networking world, the optical layer (WDM layer) appears as a
barren physical layer whose function is to transport raw bits at a high bit rate
with negligible loss. Most conventional network layer planners do not care about
the heuristics of the optical layer.

However, such lapses can often be catastrophic. Until the bit rate and the
transmission distance is under some bounded constraint (for example, small
networks), it is often not important to consider the optical parameters.

However, as the bit rate increases and transmission length increases, these
optical parameters have the capability of playing truant in the network. A
network planner must consider the affecting parameters and build a network that
accommodates the impairments caused by the optical parameters. This chapter
explores some of the design constraints involved in the WDM network design.

Consider an optical signal as a slowly varying signal of amplitude A (τ,
t) (function of distance 'τ' and time 't'), on which
various parameters are acting at all times. An optical signal, as discussed in
Chapter 1, "Introduction to Optical Networking," propagates through a
silica fiber with propagation constant β, whose value is obtained from the
solution of the basic wave equations. Further, this optical signal is subjected
to attenuation, which by virtue of itself, is a property of the propagating
mediumsilica fiber in this case. Attenuation in a fiber is characterized
by the attenuation constant α, which gives the loss (in dB) per traveled
km.

Why is attenuation parameter important? First, common thinking says that if
the total accumulated attenuation is greater than the signal input launch power
Pin,a signal will not exist at the receiving end. This,
although colloquial, is an important issue for verifying signal reception at the
receiving end of a communication channel. Second, for optical communication to
happen, a receiver (essentially a photodetector, either a PIN or APD type) needs
a minimum amount of power to distinguish the 0s and 1s from the raw input
optical signal.

The minimum power requirement of the receiver is called the receiver
sensitivity, R, and is covered in Chapter 2, "Networking with
DWDM-1." Here, we must ensure that the transmit power is high enough so
that it can maintain signal power > R at the receiver end, despite the
attenuation along the transmission line. That does not mean that if we increase
the transmit power to a high level, we can send bits across great distances.
High input power also is a breeding ground for impairments (nonlinearities such
as cross-phase modulation [XPM], self-phase modulation [SPM], four-wave mixing
[FWM] and so on). In addition, an upper limit exists for every receiver (APD
type or PIN type) for receiving optical power. This is given by the dynamic
range of the receiver, and it sets the maximum and minimum power range for the
receiver to function. For example, 7 dBm to 28 dBm is a typical
dynamic range of a receiver. Therefore, the maximum input power that we can
launch into the fiber is limited. This also limits the maximum transmission
distance, L. If PinMax is the maximum input power, the transmission
distance is L, and Pr is the minimum receiver power; then Equation
4-1 shows the maximum input power that can be sent into the fiber and Equation
4-2 shows the maximum transmission distance.

Equation 4-1

Equation 4-2

NOTE

The optical power at the receiver end has to be within the dynamic range of
the receiver; otherwise, it damages the receiver (if it exceeds the maximum
value) or the receiver cannot differentiate between 1s and 0s if the power level
is less than the minimum value.

For an input power of +5 dB and a receiver sensitivity of 20 dBm at
1550 nm, the maximum transmission distance without amplification is shown in the
following equation. (Assume α = 0.2 dB/km at 1550 nm. We usually get α
from the manufacture's spec.)

(Here we neglected all other losses.)

Further in the preceding calculation, we have neglected dispersion, fiber
nonlinearities, polarization, spectral broadening, chirp (source broadening),
fiber plant losses (connecters, splices, and aging factors), and so on. If we
consider these effects, then the maximum length is reduced further. How can we
then have ultra long-haul intercontinental systems? By placing repeaters in
cascade, we can enhance the transmission distance.

Two kinds of repeaters exist: opto-electro-opto (OEO) electrical repeaters
that detect, reshape, retime, and retransmit (3R) the signal
(channel-by-channel), and the fiber amplifiers (1R)(doped fiber, Raman, and SOA)
that boost the signal power level (no reshape and no retiming) entirely in the
optical domain. A third technique also exists: reshape and reamplify (2R)
regeneration. This technique is gaining in popularity due to its protocol
independence. This book discussed 2R in Chapter 2, so it is not necessary to
consider it here from a design perspective because it proposes a generic
alternative to the other design schemes.

Electrical repeaters have an advantage in that they can completely relaunch
the signal by regenerating and further retransmitting it due to opto-electronic
conversion and regeneration. To do so, the composite WDM signal needs to be
fully demultiplexed, which is neither cost effective nor efficient. Optical
amplifiers alleviate that problem by amplifying all the channels together completely
in the optical domain; therefore, optical amplifiers can enhance the transmission
distance. So, does that mean that optical amplifiers can increase the amplifying
distance as much as they wants? Not really! Amplifiers come at a price and induct
a trade off; they enhance the signal power level, but at the same time, they
add their own complement of noise. This noise is amplified spontaneous emission
(ASE), which was introduced in Chapter 3, "Networking with DWDM -2."
Please refer to Figure
4-1.

Amplifier noise is a severe problem in system design. A figure of merit here
is the optical signal-to-noise ratio (OSNR) requirement of the system. The OSNR
specifies the ratio of the net signal power to the net noise power. It is a
ratio of two powers; therefore, if a signal and noise are both amplified, system
OSNR still tells the quality of the signal by calculating this ratio. System
design based on OSNR is an important fundamental design tool.

NOTE

OSNR is not just limited to optical amplifier-based networks. Other active
and passive devices can also add noise and create an OSNR-limited system design
problem. Active devices such as lasers and amplifiers add noise. Passive devices
such as taps and the fiber can add components of noise. In the calculation of
system design, optical amplifier noise is considered the predominant source for
OSNR penalty and degradation. That does not imply unimportance to other sources
of OSNR penalty.

Figures 4-1 and
4-2 shows the effect of noise on signal as the signal and noise pass through
the amplifiers.

Dispersion, mentioned in Chapter 1, causes pulse spreading. The most
important form of dispersion is group velocity dispersion (GVD). Group velocity
is inversely proportional to the rate of change of propagation constant β
with respect to frequency. Isn't β a constant? Not really! β
actually (indirectly) depends on γ, the nonlinear coefficient, and
P, the power of the signal. β further depends on the group index,
which in turn depends on the GVD parameters. Therefore, dispersion causes severe
pulse spreading and leads to intersymbol interference (ISI). The GVD parameter
β2 is the second order differential of the β with respect
to change in optical frequency-omega.

Techniques are available to compensate dispersion. Note here that the
dispersion discussed so far (GVD) is called chromatic dispersion as opposed to
other forms of dispersion, such as polarization mode dispersion, or PMD. (PMD is
discussed from a design point of view in Chapter 5, "WDM Network Design
-2".) Dispersion flattened or shifted fibers are an example. Dispersion
shifted fibers (DSFs) have the zero-dispersion wavelength shifted into the
operating band. Further dispersion-compensating fibers can be placed at
strategic locations in a network so that we can reshape the broadened pulse as
desired. Yet another technique is to use fine fiber Bragg gratings (FBGs)-based
dispersion compensators. The question still remains: In a network, where do we
place the dispersion compensators? Dispersion compensation is needed only for
signals above a certain bit rate.

Another design issue is polarization. Assuming fibers to be polarization
preserving is not a good idea. Different polarization states create different
levels of PMD. PMD compensation and placement is yet another strong issue at
high bit rate signals.

Last but not least, we need to consider fiber nonlinearities. Self-phase
modulation and cross phase modulation are two common coupling problems. FWM,
stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS) are
also high bit rate, high power issues.

A system design can be optimized by considering these effects in a strategic
manner. The sections that follow consider several steps that need to be
considered in an ideal system design case. Initially, let's assume a
point-to-point link and then specifically look at ring and mesh networks in
Chapter 5.